Software Engineering • name coined at the NATO Science Committee Conference, October 1968 • Engineering-- established, scientifically sound practices that well-trained practitioners follow • Software Engineering-- the application of scientific knowledge to the the development and maintenance of software systems • Software-- ALL associated artifacts to assist with the development, operation, validation, and maintenance of programs/software systems • e.g., code, documentation, designs, requirements, user manuals, installation manuals, test cases, test results, trouble reports, revision history, installation scripts,...
Another definition • Ghezzi: A field of computer science that deals with the building of software systems that: • are so large & complex to require teams of developers • exist in multiple versions • used for many years • undergo changes/evolution
Why engineer software? • Impact on Society • Economics • Quality Concerns
Why engineer software? Impact on Society • If you fly, your life depends on software • Airbus • Your bank account depends on software • New York bank reconciliation failure • Medical devices are controlled by software • Therac-25 … and so on •
Why engineer software? Economics • An important industry • Software is an important industry • Worldwide competition • Global development models • Significant installed software base • Well-designed software easier to maintain • Poorly designed legacy s/w may be a hinderance
Why engineer software? Quality • Analogy to the automobile • U.S. automobile industry use to be very complacent about quality • Lost a significant amount of market share • Will complacency about s/w quality lead to the same result? • There are many recalls for automobiles • Some fixed for free • There are many defects in software • Some “fixed” for free • Some fixed in the “the next” release • With the customer paying for the upgrade
Quality Issues • Software is now an integral part of every facet of our societal infrastructure • Transportation • Communication • Financial • Poor quality software menaces the maintenance of that infrastructure • Software is the "Grand Enabler" holding the key to scientific and engineering challenges • Human genome project • Space exploration • Weather prediction
The nature of software • Software is a complex, intricately interconnected data aggregate • Software Development is the process of creating such a complex product, while continuously assuring that it remains consistent • Software Engineering combines some of the approaches of classical engineering with some of the abstract approaches of mathematics
Hardware versus Software • Percentage wise, hardware costs are decreasing and software costs are increasing • Is hardware development done better than software development? • Yes, but... • s/w systems tend to be more complex • tend to do new applications in s/w and well-understood applications in h/w • despite the use of more rigorous and systematic processes, hardware systems fail too
Hardware / Software Cost trends for projects S/w costs H/w costs
Trends in Software Expansion (Bernstein, 1997) Projection 1000 638 475 142 113 Expansion 100 81 75 Factor 47 37.5 The ratio 30 of machine 15 lines of code to a 10 source line of code 3 Order of Magnitude Increase Every Twenty Years 1 1960 1965 1970 1975 1980 1985 1990 1995 2000 Machine Macro High Level Database On-line Prototyping Subsecond Object Large Scale Instructions Assembler Language Manager Time Oriented Reuse Sharing Programming Regression Small 4GL Testing Scale Reuse
What is novel about software? • product is unprecedentedly complex • application horizons expand very fast--with human demands/imagination • construction is human-intensive • solutions require unusual rigor • extremely malleable--can modify the product all too easily
How to improve Software Quality • Treat software as a PRODUCT produced in a systematic way according to a well-defined PROCESS designed to achieve explicit quality objectives • Build quality in • Define software product • Reason about the product • Incorporate validation as integral steps in the process
But what is that process? • What methods should be used? • What tools support those methods? • How do we know that these methods and tools will lead to a better product? Based on experimentation, build up a sense of what are the best practices
Software Lifecycle requirements reqts. analysis design specs validation coding validation testing adequacy maintenance revalidation
Waterfall Model • requirements-- a complete,consistent specification of what is needed • provides visibility for customers, developers, and managers • benchmark for testing and acceptance • reduces misunderstandings • requirements analysis • evaluate completeness and consistency • evaluate needs and constraints • evaluate feasibility and costs • development and maintenance costs • probability of success
Waterfall Model (continued) • design specifications--a description of how the requirements are to be realized • high-level architectural design • low-level detailed design • design validation • traceability between requirements and design decisions • internal consistency
Waterfall Model (continued) • code--realization of the design in executable instructions • code validation • assure coding and documentation standards have been maintained • internal consistency • e.g., syntactic analysis, semantic analysis, type checking, interface consistency • consistency between design/requirements and code
Waterfall Model (continued) • testing--reveal problems, demonstrate behavior, assess reliability, evaluate non-functional requirements (e.g., performance, ease of use) • unit testing • integration testing • system testing • acceptance testing • regression testing • testing validation • adequacy of the testcases
Waterfall Model (continued) • maintenance--the process of modifying existing software while leaving its primary functionality intact • corrective maintenance-- fix problems (20%) • adaptive maintenance-- add new functionality/enhance existing features (30%) • perfective maintenance-- improve product (50%) • e.g., performance, maintainability • 3 primary steps • understand existing software • change existing software • revalidate existing software • maintenance involves all the previous phases of the lifecycle
Is the waterfall model an appropriate process model? • recognizes distinct activities • clearly oversimplifies the process • wait, wait , wait, surprise model • actual processes are more complex • numerous iterations among phases • not purely top down • decomposition into subsystems • many variations of the waterfall model • prototyping • re-engineering • risk reduction • ...
Software costs • Development costs • generally measured in hundreds to thousands of dollars per delivered LOC • many artifacts associated with a line of code • testing and analysis is usually 50% of this cost • Maintenance costs • 2-3 times as much as development
Software Costs Development costs reqts and design code 35% 15% testing 50% Full lifecycle costs code testing reqts/design maintenance
Decisions made throughout a project affect the cost of maintenance • planning for maintenance increases front end cost • Industry is often unwilling to pay these costs up-front • Time to market • Job turn-over
Some interesting numbers • About 25% of s/w projects fail • Failure rate increases as the size of the project increases • Costs about $100/LOC • Ranges between $10-$600 • Typical programmer produces about 30 LOCs a day • Ranges between 10-100 LOCs • Ranges between 3-10 faults/KLOC
Barriers to engineering software • industry’s short term focus • shortage of skilled personnel • inadequate investment in R&D • PITAC (Kennedy-Joy) Report • US SW GNP is ~$228B but less than 1% spent on R&D • Poor technology transfer models • "toss over the fence" • Lack of “good” standards • Lack of experimental basis for standards
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